Cotton is an important and valuable field crop primarily due to the intrinsic value of their fiber (lint) to provide soft, breathable textile products. Cotton is also an important source of vegetable oil used extensively in foodstuffs for baking and frying and in spreads such as margarine and mayonnaise. The seed bagasse is used as raw materials in livestock feed, fertilizer, paper, and biofuel. Despite the importance of cotton's secondary products, 90% of cotton's value resides in the lint fiber.
Commercial cotton breeding programs have aimed to develop new, unique, and superior cotton varieties with desirable traits such as higher fiber (lint) yield, earlier maturity, improved fiber quality, resistance to diseases and insects, tolerance to drought and heat, and other improved agronomic traits. However, breeding cotton for yield and fiber quality has been challenging. Part of the challenge comes from limited genetic diversity in breeding programs and increasing vulnerability of germplasm to environmental stress. Among all types of environmental stress, drought or water deficit is a major limiting factor for cotton production in many cotton production areas.
Studies on the response of cotton to drought stress have shown many deleterious effects of drought, depending on the time, length, and severity of the stress as well as the plant developmental stage. Drought stress in cotton can decrease leaf water potential and leaf area, and therefore promotes stunted vegetative growth including reduced shoot growth, shortened internodes, and abscised lower leaves when the stress is severe. If the drought stress is encountered during the reproductive stage, it increases square and boll abortions, leading to lower yield.
The detrimental effects of drought can be minimized by the development of drought tolerant cotton cultivars. However, there are limited reports on this aspect due to the complex nature of drought tolerant mechanisms. Cotton possesses sophisticated mechanisms to adapt and grow in soils with limited water availability. For instance, cotton develops a deep-penetrating and extensive root system (having large numbers of lateral roots) with narrower tap roots, sheds leaves and fruits, and has a flexible fruiting period when exposed to drought stress. Differences in stomatal distribution and behavior have been observed in cotton grown in soils with restricted water availability. Despite these phenotypic manifestations of drought tolerance, little is known about molecular mechanisms underlying drought resistance in cotton.
There is a need to breed new cotton varieties with improved drought tolerance. New cotton germplasm providing drought tolerance is highly sought after. To assist molecular breeding, genetic loci and markers, haplotypes, and chromosomal intervals that confer or are linked to drought tolerance are also much desired. Further, there is a need for a rapid, cost-efficient method to assay, monitor, and introgress drought tolerance traits in cotton.